Traditionally, hematological studies have relied upon morphological and histochemical techniques except in the case of the reticulocyte which has been a favorite target for studies of gene structure and function by recombinant DNA techniques. We are studying a system in which recombinant DNA techniques can be used to study the much more complex series of differentiation steps leading from promyelocytes to either granulocytes or macrophages. Using an "immortal" human leukemic cell line which can be induced to differentiate in vitro, we have already constructed libraries of clones of genes expressed at different developmental stages of acute myeloid leukemia. Many genes which are strongly regulated have been isolated. The system has a unique set of advantages for the discovery of what control signals are active during hematopoietic differentiation and the direct relation of their structure and function. During the past year we have conducted detailed analyses of a substantial group of cDNA clones for regulated genes and have used these clones as probes to isolate the corresponding human chromosomal clones from a phage lambda library. Recently, we have started experiments to reinsert tagged versions of these genes into HL60 cells in order to identify regulatory sequences. We have also used 2D protein gel techniques to determine a detailed time course for the many changes occurring during differentiation of these cells.